REPEATED CHANGE FROM CRUSTAL SHORTENING TO OROGEN-PARALLEL EXTENSION IN THE AUSTROALPINE UNITS OF GRAUBUNDEN

The structural analysis of the Austroalpine units in Graubunden reveal s the existence of two orogenic cycles, Cretaceous and Tertiary in age , both including thrusting followed by extensional overprint. Such ext ensional faulting occurred in the Late Cretaceous and in the Early to Mid-Oligocene. In both episodes, the direction of extension was parall el to the strike of the Alpine chain. Five stages of the tectonic evol ution are recognized: (1) Late Cretaceous nappe imbrication and sinist ral transpression (Trupchun phase). The Austroalpine nappe pile was as sembled by oblique east-over-west imbrication of the northwest margin of the Apulian microcontinent and was affected by sinistral transpress ion, localized in the east-west trending Albula steep zone. (2) Late C retaceous extension (Ducan-Ela phase). The nappe pile was overprinted by top-east to top-southeast directed normal faulting. Recumbent folds developing simultaneously with and in the footwalls of large low-angl e normal faults reflect east-west extension and vertical shortening of initially steeply oriented layers. Normal faulting and recumbent fold ing are therefore viewed as different expressions of the same extensio n process. (3) Early Tertiary collisional deformation (Blaisun phase). The Late Cretaceous extensional features were overprinted by east- to southeast-striking folds which are probably coeval with Early Tertiar y northward thrusting of the entire Austroalpine nappe pile (''orogeni c lid'') over the deeper Penninic units. (4) Early to Mid-Oligocene ex tension (Turba phase). This second episode of east-west extension affe cted the boundary zone between the orogenic lid and the underlying Pen ninic units, locally leading to an extensional decoupling between thes e two levels (Turba mylonite zone). (5) Late Oligocene post collisiona l shortening (Domleschg phase). The direction of shortening changed to NW-SE at this time. On the scale of the entire Austroalpine realm, bo th Cretaceous shortening and following Late Cretaceous extension migra ted westward. This first extensional event is therefore interpreted to be caused by instability of a westward advancing orogenic wedge. In c ontrast, Tertiary east-west extension was contemporaneous with ongoing north-south shortening. The recognition of two orogenic cycles contra dicts the classical view that the Alpine orogeny involves a continuous tectonometamorphic evolution from Cretaceous subduction and high-pres sure metamorphism to Tertiary exhumation and Barrow-type metamorphism. Instead, it is postulated that nappe formation related to subduction and exhumation associated with extension occurred twice during the Alp ine orogeny.